Dual mode telephone subscriber loop current detector

ABSTRACT

A detector circuit is shown for detecting off-hook and dial pulse signals in a telephone system which, under the control of a ringing relay, reconfigures itself to provide ring-trip detection. The detection circuit includes a bridge which is responsive to circulating loop currents, while at the same time unresponsive to longitudinally balanced currents, and a two-stage voltage level detector circuit. An RC timing circuit is included between the two stages to insure that the detector is unresponsive to short duration pulses and, at the same time, once energized, will bridge over short duration interruptions of detected pulses.

This application is a division of application Ser. No. 763,288, filed onJan. 28, 1977, now U.S. Pat. No. 4,087,646.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to signal detector circuits and, moreparticularly, to sensitive detectors for telephone loop signaling.

2. Description of the Prior Art

It is common practice to utilize sensitive relays to detect signalingcurrents in telephone subscriber loops. Thus, when a subscriber goesoff-hook, the resulting current flow in the subscriber loop causes aline relay to operate and signal the central office of the off-hookcondition. Similarly, loop interruptions caused by a rotary dial aredetected in a dial pulse receiver, also including a sensitive relayresponsive to interruptions in loop currents.

On the other hand, for subscribers who are being called rather thaninitiating a call, ringing signals are applied to the subscriber's loopto operate the telephone ringer. When the called subscriber goesoff-hook, the resulting loop current flow is detected by a ring-triprelay to interrupt the ringing signals on the loop. This function,called "ring-trip," requires the detection of a low level dc loopcurrent in the presence of a high level ac ringing signal.

Electronic loop current detectors, such as that shown in the copendingapplication of S. J. Brolin and S. Colodner, now U.S. Pat. No.4,056,690, granted Nov. 1, 1976, and assigned to applicant's assignee,are known. Adequate discrimination against dial pulse splitting andadequate threshold stability remain problems, however.

In many subscriber loop carrier systems, such as those shown in J. L.Caldwell U.S. Pat. No. 3,963,869, granted June 15, 1976, and thecopending application of T. N. Rao-R. Toumani, now U.S. Pat. No.4,028,628, granted June 7, 1977, telephone supervisory and loop closuresignals cannot be transmitted directly through the carrier system. Thesesignals must therefore be detected at the remote end of the carriersystem and transmitted through the system to the central officeterminal.

The usefulness of such carrier systems is dependent, in part, on how farthe subscriber loops can be extended from the remote end of the system.In order to extend such loops beyond the remote terminal for substantialdistances, a sensitive and discriminating detector for off-hook, dialpulse and ring-trip currents is required. Moreover, the detector mustprovide accurate detection of these signals without significantlydistorting the dial pulses.

SUMMARY OF THE INVENTION

In accordance with the illustrative embodiment of the present invention,off-hook, dial pulsing, and ring-trip signals are all detected by acommon circuit. Under the control of ringing relay contacts, the circuitis configured as a loop current detector to detect off-hook and dialpulses in the absence of ringing signals. When the ringing relaycontacts are operated, ringing is applied to the subscriber lines andthe detection circuit is simultaneously reconfigured to provide asensitive ring-trip detector.

The common portion of the ring-trip, off-hook, and dial pulse detectorcircuit has two stages. With the ringing relay released, the first stagedetects metallic flow in the loop and its output faithfully follows allof the transitions in such loop current, including unwanted split dialpulses and noise pulse transistions. Inserted between the first andsecond stages of the detector is a timing circuit which, in combinationwith an automatically switched threshold in the second stage, insuresthat the operate and release transitions of the second stage cannotfollow current transitions occurring at a substantially faster rate thanstandard dial pulse transitions. This rate is selected to block induced60 Hz signals, noise pulses and to bridge over split dial pulses.

During ringing, circuit components are rearranged to provide a low-passfilter for the ringing signals, thus preventing these signals frominterfering with the detection of direct current ring-trip signals.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a detailed circuit diagram of a loop closure and ring-tripdetector in accordance with the present invention;

FIG. 2 is a simplified circuit diagram of the detector of FIG. 1configured as a loop closure detector; and

FIG. 3 is a simplified circuit diagram of the detector of FIG. 1configured as a ring-trip detector.

DETAILED DESCRIPTION OF THE DRAWING

In FIg. 1 there is shown a detailed circuit diagram of a detectorcircuit which can be used both for detecting loop closure in a telephonecircuit connected to ring conductor 10 and tip conductor 11 on callsinitiated by the subscriber, and can also detect ring-trip signals forcalls initiated at other subscribers and intended for the subscriberconnected to conductors 10 and 11. The detector circuit of FIG. 1 isintended for use with telephone subscriber carrier systems in which aportion of the loop between the subscriber and the central officeincludes a carrier system employing either frequency or time divisionmultiplexing techniques. A time division multiplexing system of thistype is shown in the aforementioned J. L. Caldwell patent while ananalog carrier system is disclosed in the aforementioned copending U.S.Pat. No. 4,028,628, of T. N. Rao-R. Toumani. In either case, it isnecessary to detect supervisory and signaling currents at or near thesubscriber location and to translate these currents into a form suitablefor transmission over the carrier facilities. Winding 12 of hybridtransformer 13 is the input port for voice signals derived from thecarrier system. Similarly, winding 14 is the output port from hybridtransformer 13 to the carrier system. Windings 15 and 16 couple thesevoice signals to and from the subscriber connected to ring conductor 10and tip conductor 11. The balance of the circuit of FIG. 1 comprises adetector circuit for currents flowing through the subscriber telephoneset.

The carrier system also delivers a ringing signal indication to aringing signaling detector 17 which, when energized, causes relay driver18 to operate K1 relay 19 from voltage source 20. A suitable ringingsignal encoder and decoder for this purpose is shown in the copendingapplication of R. J. Canniff-M. T. Manfred, now U.S. Pat. No. 4,048,448,granted Sept. 13, 1977.

The detector of FIG. 1 is a dual-mode detector which, in one mode,detects off-hook and dial pulsing signals from the subscriber station.In the other mode, the detector of FIG. 1 detects ring-trip currentswhich flow through the subscriber set when it is lifted off-hook inresponse to a ringing signal at the subscriber station. Contacts on K1relay 19 reconfigure the detector of FIG. 1 between these two modes ofoperation.

The detector circuit comprises a capacitor C3 which completes thealternating current path to the subscriber set and permits directcurrent source 21 to provide talking current through resistor R_(R),winding 15, ring conductor 10, the subscriber telephone set (not shown),tip conductor 11, winding 16, resistor R_(T) to ground. The detectorcircuit also includes a bridge circuit including resistors R1, R2, R3,R4, and R5 connected in shunt with current detecting resistors R_(R) andR_(T). Under the control of K1 contacts 22, capacitor C2 can besubstituted for resistors R3 and R5 in the circuit. K1 contacts 23provide a short circuit across winding 16 and K1 contacts 24 transferring conductor 10 from winding 15 and the detector circuit to a ringingsupply 25. K1 contacts 22, 23 and 24 are, of course, under the controlof K1 relay 19.

With K1 relay 19 unenergized, the normally open portions of K1 contacts22 and 24 remain open and the resulting bridge circuit has theconfiguration shown in FIG. 2. In FIG. 2 the components of the circuithave been identified with the same reference numerals as are used inFIG. 1. It can be seen in FIG. 2 that this bridge circuit provides anoutput which is sensitive to metallic currents flowing circularly aroundthe loop but is insensitive to longitudinal currents which flow in thesame direction through both legs of the loop. That is, the voltage(V2-V1) is responsive to differential polarity voltage drops acrossresistors R_(R) and R_(T), but is insensitive to voltage drops ofsimilar polarity across these two resistors. When voltage V2 rises aboveV1 due to this differential voltage drop, the balance of the detectorresponds to this voltage condition in a manner which will be describedin connection with FIG. 1.

Returning then to FIG. 1, it is seen that the detector includes a firststage 40 including comparator 26 to which voltages V1 and V2 areapplied. Resistor R6 provides feedback around comparator 26. Resistor R7supplies voltage from source 27 through resistor R8 to charge capacitorC1. The voltage on capacitor C1 provides one input to a secondcomparator 28, the other input of which is supplied from the voltagedivider comprising resistors R9 and R10 across voltage source 29.Comparator 28 also has a feedback resistor R11 connected between itsoutput lead 30 and a positive input to comparator 28.

In the absence of metallic loop current on conductors 10 and 11, voltageV1 is greater than voltage V2 and the open collector output ofcomparator 26 is in its high (open circuit) state, thus permittingcapacitor C1 to charge from source 27. When the subscriber lifts thehandset from its cradle, thus going off-hook, a metallic loop currentflows from source 21 through resistor R_(R), winding 15, conductors 10and 11, winding 16, and resistor R_(T). This metallic loop currentcauses comparator 26 to operate, driving its output to a low (closedcircuit) state and permitting capacitor C1 to discharge. After apreselected period of time, the charge on capacitor C1 decayssufficiently to trigger comparator 28 and thus provide a high (opencircuit) output on lead 30.

Transitions in the metallic loop current do not occur instantly due tothe inductance and capacitance of the subscriber's ringer. This tends todistort dial pulses and hence resistor R2, instead of being connected tothe upper end of winding 15, is connected to the lower end. That is,winding 15 is included, along with voltage dropping resistor R_(R), inthe detection path for voltages developed across the upper leg of thebridge circuit connected to resistors R2 and R3. Changes in loop currentdevelop a voltage transient across winding 15 which tends to compensatefor the slow rise time of metallic loop current. Thus, by includingwinding 15 in the bridge circuit, the detector is able to follow, notonly the level, but also the rate of change of metallic current and istherefore able to follow dial pulses with little or no transitiondistortion.

Under some conditions, it is possible for a current pulse to exist onconductors 10 and 11 which does not represent a valid off-hook or dialpulsing signal. These unwanted transients may, for example, arise due toconductor 10 or 11 being shorted to ground, due to the presence ofinduced 60 Hz signals on these conductors, to lightning, or to highlevel voice frequency signals on the line. Comparator 26 would tend tofollow these unwanted transients and, by itself, produce false signalingconditions. An interstage timing circuit comprising resistors R7 and R8and capacitor C1 and the second stage 50 cooperate to prevent theseunwanted indications.

In particular, when comparator 26 detects an interruption in metallicloop current, the output from comparator 26 goes to a high (opencircuit) state and capacitor C1 begins charging exponentially from 0volts dc toward approximately +5 volts dc. The time constant of thischarging circuit is selected to be approximately 13 milliseconds toaccommodate standard 10 pulses per second dialing rates. Afterapproximately 14 milliseconds, the voltage on capacitor C1 crosses thethreshold voltage V4 (3.27 volts dc) and comparator 28 switches itsoutput to a low condition. The feedback resistor R11 immediately reducesthe threshold voltage V4 to +1.24 volts dc, which threshold must becrossed before comparator 28 will switch back to its high (open circuit)output condition.

While comparator 26 continues in the high output condition, capacitor C1continues to charge, approaching +5 volts dc. A subsequent loop closure(dial pulse contacts closing) is detected by comparator 26, whichswitches its output to the low state. Capacitor C1 therefore beginsdischarging through resistor R8 and comparator 26 towards 0 volts dcwith approximately a 10 millisecond time constant. In approximately 14milliseconds, the voltage on capacitor C1 (V3) decreases to the +1.24volt dc threshold voltage V4 and the output of comparator 28 switches tothe high condition.

It can be seen that the second stage 50, including comparator 28, delaysboth the make and break dial pulse transitions equally by approximately14 milliseconds. The second stage, however, alters neither the percentbreak nor the dial pulsing speed detected by the first stage.

When the first stage 40 detects an unwanted noise transition, that is, atransition which continues for less than approximately 10 milliseconds,capacitor C1 has not had sufficient time to completely charge and switchcomparator 28 and hence such unwanted transitions do not appear atoutput lead 30. Similarly, the delay introduced by capacitor C1 and thehysteresis introduced by resistor R11 cooperate to prevent comparator 28from following 60 Hz induced signals on the telephone loop.

It can be seen that the values of resistors R7 and R8 and capacitor C1,together with the change in the threshold of comparator 28, providesubstantially equal charging and discharging times for capacitor C1 toenable and disable comparator 28. These equal times insure properlyformed and properly timed dial pulses and yet permit a delay sufficientto discriminate against noise signals on the telephone loop. Comparators26 and 28 may comprise standard open collector comparators such as thoseshown in the copending application of S. J. Brolin, now U.S. Pat. No.4,056,690, granted Nov. 1, 1977, and assigned to applicants' assignee.

When ringing signals are transmitted from the central office to thesubscriber connected to conductors 10 and 11, ringing signal detector 17detects these signals to operate K1 relay 19. A ringing detector for adigital carrier system is shown in the aforementioned copending U.S.Pat. No. 4,048,448 of R. J. Cannif-M. T. Manfred, assigned toapplicants' assignee. When thus operated, K1 relay 19 connects ringingsupply 25 to ring conductor 10 and reconfigures the detector circuit todetect ring-trip signals. In FIG. 3 there is shown a simplified circuitdiagram of the ring-trip detector configuration which results. Again, inFIG. 3 the same reference numerals have been used to identify the samecircuit components.

Turning then to FIG. 3, it can be seen that ring supply 25 is connecteddirectly to ring conductor 10 and the ringing return path is provided bytip conductor 11 and resistor R_(T) to ground potential. While thesubscriber remains on-hook during ringing, resistor R4 and capacitor C2form a low-pass filter for the ringing signals and the output voltage V1remains high. A constant reference voltage is provided by a voltagedivider including resistor R_(R), winding 15, resistor R2, and resistorR1. This reference voltage provides input voltage V2 to comparator 26 inFIG. 1.

When the subscriber goes off-hook in response to the ringing signal, adirect current component flows in the ringing return current on tipconductor 11. The voltage drop across resistor R_(T) from this directcurrent component causes voltage V1 to decrease and to eventually becomemore negative than reference voltage V2. As can be seen in FIG. 1, thiscauses comparator 26 to operate and, after a 14 millisecond delay,comparator 28 to operate to provide an output signal on lead 30. Thisoutput signal on lead 30 is a ring-trip signal and is used to terminatethe application of ringing signals to the subscriber loop.

It can be seen that the detector circuit of FIG. 1 operates both as aloop closure detector and as a ring-trip detector under the control ofcontacts on K1 relay 19. Moreover, when in the loop closureconfiguration, the detector of FIG. 1 detects dial pulses anddiscriminates against short noise pulses and 60 Hz signals on thetelephone loop.

The arrangement of FIG. 1 has one further advantage. Under somecircumstances dial pulses may be "split," that is, brief interruptionsin loop current may occur in the course of an otherwise proper dialpulse. Split pulses sometimes occur if many ringers are present on theloop, causing excessive transients in the dial pulse current, or if theloop is poorly balanced and exposed to a high level of longitudinalinduction. The two-stage detector of FIG. 1 not only delays thedetection of such dial pulses, but, due to the delay of the trailingedge of the dial pulses, serves to bridge over short interruptions inthe dial pulses. Thus the dial pulse indications on output lead 30 donot track the interruptions in such split dial pulses and insteaddelivers a well-formed dial pulse for transmission back to the centraloffice.

The sensitive detector of FIG. 1 enables the local drop from thedetector to the location of the physical subscriber to be extended wellbeyond the normal range for other types of detectors. Indeed, thedetector of FIG. 1 will detect off-hook, dial pulsing, and ring-tripsignals on loops of up to 1800 ohms resistance, plus 200 ohm stationset. This permits subscribers to be located at considerable distancesfrom the remote terminal of a carrier system. This flexibility has manyadvantages, including the ability to serve widely scattered subscriberswith a carrier system in a rural environment. Moreover, this flexibleservice is obtained with improved dial pulse detection due to theability of the detector circuit of FIG. 1 to ignore splits in dialpulses and to discriminate against short noise bursts which mightotherwise be interpreted as dial pulses. Further, the use of thetransformer primary winding 15 in the detection circuit speeds up"sluggish" transitions.

It is obvious that the principles embodied in the circuit of FIG. 1 canbe applied to subscriber loops serving more than one party. In thiscase, of course, the ringing supply 25 must be arranged andinterconnected to provide selective ringing signals superimposed onappropriate polarities of direct current voltage. Reconfiguring thecircuit reduces the number of components required for ring-tripdetection and thus reduces the cost and increases the reliability of thedetector circuits.

What is claimed is:
 1. A threshold detector comprisinga first voltagecomparator including first and second input terminals and a first outputterminal, said comparator being responsive to voltages at said firstinput terminal exceeding the voltage at said second input terminal forproducing a first output voltage at said first output terminal, saidfirst voltage comparator including a first feedback circuit connectedfrom said first output terminal to said second input terminal, acapacitive timing circuit connected to said first output terminal ofsaid first voltage comparator, and a second voltage comparator includinga third and a fourth input terminal and a second output terminal, saidcomparator being responsive to the output of said timing circuitconnected to said third input terminal exceeding a second thresholdvoltage at said fourth input terminal for producing a second outputvoltage at said second output terminal, said second voltage comparatorincluding a second feedback circuit connected from said second outputterminal to said fourth input terminal.
 2. A threshold detectoraccording to claim 1 wherein said capacitive timing circuit comprisesacapacitor, and means for charging and discharging said capacitor forsubstantially equal periods of time.